An alkaline secondary battery is widely prevailing as a power source for portable electronic equipment, and is furthermore put into practical use as a power source for HEV (a so-called hybrid car).
With such prevalence, in recent years, further technical advancement in performance has been required for the alkaline secondary battery. Specifically, enhancement in performance of maintaining residual capacity by inhibition of self discharge, enhancement in charge efficiency (especially enhancement in charge efficiency at high temperatures), and the like have been required.
For inhibition of self discharge and enhancement in charge efficiency, in order to inhibit the oxygen evolution reaction which is a side reaction on recharging an alkaline secondary battery, the oxygen evolution potential (sometimes referred to as “oxygen overvoltage”) is required to be raised.
As one technique for making the oxygen evolution potential shifted to the noble side (raised), for example as described in the following Patent Document 1, the technique using a positive electrode material for an alkaline secondary battery prepared by adding a rare earth element such as holmium (Ho) to an active material composed mainly of nickel hydroxide with a surface coated with a cobalt compound is known.
By using a positive electrode material added with a rare earth element as described above, the effect of inhibiting self discharge and enhancement in charge efficiency have been confirmed.